Mobile subscribers use wireless communication devices to communicate over a cellular wireless communication network by transmitting and receiving wireless signals between the wireless communication devices and one or more base stations, located at varying distances proximate to the various mobile subscribers.
Changing conditions, including movement of the mobile subscribers relative to the one or more base stations, will often necessitate that the wireless communication devices identify a different base station for facilitating further communication within the network. A decision to select a new serving base station, based upon the movement of the mobile subscriber relative to the one or more base stations, is often the result of a change in the received signal strength between the wireless communication devices and the previous serving base station. Either the received signal strength between the previous serving base station has degraded to an unacceptable level and/or a stronger signal is now available between the wireless communication device and a new alternative base station.
To insure that the wireless communication device is consistently communicating with the best available serving base station, a wireless communication device will periodically scan for neighboring cells and corresponding base stations. When appropriate, a communication device will initiate a reselection or a handover. A handover occurs when a new serving base station is selected, while the wireless communication device is in dedicated mode (e.g. when a call is in progress). A reselection occurs when a new serving base station is selected, while the wireless communication device is in idle mode. A similar selection process will also take place when power is initially applied to the wireless communication device, and the wireless communication device is initially locating itself within the network.
Medium-sized band GSM systems, corresponding to the Global System for Mobile Communications, generally provide 200 kHz carrier separation between channels. However, given the power profile of a typical signal, spilled power can occur in several sequentially adjacent channels at various attenuation levels. The GSM specification defines a reference interference level for several sequentially adjacent channels, which corresponds to a detectability level and the allowable amounts of adjacent channel power. In the case of an immediately adjacent channel or a channel 200 kHz away, the GSM specification provides a reference interference level that is less than −9 dB, relative to the power transmitted on the primary channel. For adjacent channels 400 kHz and 600 kHz away, the GSM specification requires that the reference interference level not exceed −41 dB and −49 dB, respectively.
The GSM specification further provides a detection margin of 9 dB. This translates to a limit of spilled power in the amounts of −18 dB in an immediately adjacent channel, and requires that the spilled power not exceed −50 dB and −58 dB for channels 400 kHz and 600 kHz away, respectively.
In some instances, the spilled power in an adjacent channel can have a power level, which is sufficiently strong to be observable by the wireless communication device. For example, if a primary signal was received by the wireless communication device with a signal strength of −41 dB, the immediately adjacent channel might have a power level as seen by the wireless communication device, which is substantially comprised of spilled power, that has a power level of −59 dB, and still be within the limits for spilled power allowed in the specification. Furthermore, a channel having a power level of −59 dB is sufficiently strong, so as to be identified as a channel having a potentially detectable signal. Generally, to have sufficient power for receiving a signal on a particular channel, the signal needs to have a signal strength greater than −104 dBm, also known as the reference sensitivity level. In the same example, spilled power can produce a channel power level of −91 dB and −99 dB in the adjacent channels 400 kHz and 600 kHz away, respectively, that is seen by the wireless communication device. At −91 dB and −99 dB, both of the channel power levels are similarly sufficiently strong to be identified as channels, which potentially have signals of interest.
Preferably, standard frequency planning will provide that nearby channels will not be used in the same geographical area, in order to minimize the effects of interfering spilled power from an adjacent channel. However sometimes, even if the nearby channels have not been used, the system can still include the nearby channel in a list of defined neighboring cells that is transmitted by the serving cell, which is then used to monitor and search for neighboring cells. In these instances, a wireless communication device can sometimes spend time and energy trying to acquire a signal on a nearby channel, where the power measured for the channel is substantially comprised of spilled power from an adjacent channel, and where there is little hope of acquiring a signal. A wireless device can sometimes make several attempts to acquire a signal before exceeding the limit on the allowed number of attempts and attempting to acquire the next channel in the list.
Consequently, there is a need for a method and apparatus for scanning neighboring cells, which can eliminate from consideration and from the list of channels to be scanned, channels where the detected power levels are determined to be substantially comprised of adjacent channel power.